It depends on the representation of your impedance spectra and the frequency range. If you plot Im(Z) vs. omega graph and you have a maximum (not always of course observable), the maximum frequency corresponds a relaxation time (t=1/omega). There are another representation like Bode diagram where it can be derived from the break points. Try to find the following book comprehensive :"Impedance Spectroscopy" by J. Ross Macdonald.